The present invention relates to a non-aqueous electrolyte secondary battery, and in particular to a non-aqueous electrolyte secondary battery in which positive and negative electrodes are formed of a composite metal oxide including Li and carbon, respectively. New portable electronic devices such as camcorders, portable cellar phones, and laptop computers have been successively developed. As efforts have been made to provide compact and light weight devices, the need for portable batteries having a higher energy density has arisen.
Aqueous solution type batteries such as lead batteries and nickel-cadmium batteries have been primarily used as secondary batteries. Although these batteries exhibit excellent cycle characteristics, they are not satisfactory in term of their energy density, etc. Further, such batteries are problematic as far as protecting the environment is concerned. Development of a secondary battery which can replace these batteries has arisen.
From such circumstances, development of a non-aqueous electrolyte secondary battery (so-called lithium secondary battery) which is pollution-free and has a high energy density due to a high operating voltage has been of great interest.
Since the energy density of the non-aqueous battery depends upon the characteristics of its cathode a number of positive electrodes have been proposed, analyzed and studied.
Success of the development of the secondary battery depends upon whether a lithium battery exhibiting excellent cycle characteristics can be developed.
However, it may be concluded that the satisfactory results of the development of the lithium negative electrode are very few from these points of view.
For example, although a lithium secondary battery having an "AA" size, using lithium for the negative electrode having excellent characteristics has been proposed and introduced, a number of difficulties have been encountered with the lithium negative electrode that have not yet been solved.
In other words, in the non-aqueous electrolyte battery using lithium metal or lithium alloy for the negative electrode, lithium becomes inactive and is deposited under a powdery condition by repetition of charge/recharge cycles. During charging, crystals of a lithium will grow in a dendrite manner and pass through micro-pores of a separator membrane, or spacings, between fibers of a separator's unwoven fabric to reach a positive electrode, causing an internal short. Accordingly, a satisfactory lifetime of such a battery through repeated charge/recharge cycles can not be obtained. Since the activity of metallic lithium is very high, safety problems have not been solved. Li-Carbon intercalation compound (hereinafter referred to as LI-CIC) electrode has been developed as a material for a negative electrode which may replace a lithium negative electrode. It is deemed that this LI-CIC electrode has a favorable lifetime through repeated cycles. Since the Li-CIC in which carbon is intercalated with lithium ions may take part in a reversible oxidation/reduction reaction involving an electrochemical undoping/doping of lithium ions in an organic electrolyte containing lithium salt, and the oxidation/reduction potential falls in the range of about 0.02 to 1.0 volt, the Li-CIC may provide an excellent negative electrode material for the non-aqueous electrolyte secondary battery if it is used with a proper positive electrode material. Specifically, in a battery system in which the negative electrode is formed of the LI-CIC, the lithium ions which have been doped in the negative electrode carbon migrate to the positive electrode and play a role to guide, in the positive electrode, electrons which come from the negative electrode via an external circuit during discharge. During charge, the lithium ions which have migrated to the positive electrode will return to the negative electrode and play a role to guide, in the negative electrode, the electrons which return via the external circuit. Since no metallic lithium exist in the battery in any charge/discharge cycle, deposition of inactive lithium and growth of dendrite will never occur. Since the crystal structure of the positive and negative electrode activating materials is resistant to breakdown, excellent charge/discharge cycle characteristics can be obtained.
On the other hand, the characteristics of the organic electrolyte used are very important to obtain excellent charge/discharge cycle characteristics in the non-aqueous electrolyte secondary battery. To this end, a number of studies have been made on the relationship between the characteristics of the organic electrolyte and the charge/discharge cycle characteristics and findings which will be described hereafter have been obtained.
1. The conductance of the organic electrolyte is remarkably improved by a combination of a solvent having a high dielectric constant and a solvent having a low viscosity. This can be semi-quantatively explained by the dissociation and mobility of ions in the electrolyte.
2. As the conductance of the electrolyte becomes higher, the polarization of the lithium negative electrode becomes less and the charge/discharge efficiency becomes higher.
3. A system in which propylene carbonate and sulfolane or dimethylsulfoxide are used as a solvent having a high dielectric constant is mixed with 1,2-dimethoxyethane and a solvent having a low viscosity gives a high electric conductance and excellent lithium charge/discharge performance.
However, it has been found f rom the present inventors' study that when an electrolyte containing, as an organic solvent, a mixture solvent of propylene carbonate and 1,2-dimethoxyethane is used in a non-aqueous electrolyte battery having a negative electrode formed of a Lithium-CIC, the battery exhibits a charge/discharge cycle which is excellent, to some extent, at room temperature while there is a disadvantage that the capacity is rapidly lowered and the cycle life time is shortened to about 1/10 of that at room temperature if charge/discharge cycles are repeated at an elevated temperature (for example, 40.degree. C.).
The secondary batteries which can replace the existing Ni-Cd or lead batteries should, of course, be capable of working temperatures ranging from at least -20.degree. C. to above 40.degree. C.
Therefore, rapid lowering of the capacity of the non-aqueous electrolyte secondary battery having a negative electrode formed of LI-CIC in a high temperature environment is a great obstacle against practical use.